In numerous industrial, aerospace, and research applications, there is a need to monitor the temperature of a particular process or component. For example, the knowledge of the temperature enables one to determine whether a particular process used in the application is functioning in an optimized manner. Knowledge of the temperature can also enable one to prevent the overheating of a component and any damage that may result, should the temperature exceed a maximum level at which the particular component can operate.
There has been a growing need for temperature sensors with very high temperature sensing capability. This need is especially evident in the development of gas turbines for both jet engine and power generation applications. Conventional temperature sensors such as thermocouples and pyrometers are not suitable for this purpose. As a result, a number of novel temperature sensors have been developed in recent years to meet this need.
One of these approaches is based on the temperature dependent fluorescence decay time found in certain materials. Fluorescence is a form of photoluminescence wherein a molecule, ion or atom at its lowest energy, or ground state, is promoted to a higher energy state, called an excited state, by absorbing radiant energy. The actual fluorescence emission is produced when the excited species undergoes a radiative transition from the excited state to a lower state, usually the ground state. The emitting state may be either directly excited through the absorption of a photon by a ground state specie or indirectly after subsequent cascade steps.
In fluorescence based temperature sensing, the most commonly found type of fluorescence decay has an exponential dependence on time according to the following relationship: EQU I(t)=I.sub.o e.sup.-t/.tau.
wherein I(t) is the intensity of the fluorescent radiation at time t after the termination of the exciting radiant energy (including cascade of energy transfer steps if present), I.sub.o is the intensity of the fluorescent radiation at t=0, and .tau. is the temperature dependent decay time of the fluorescent emission. With this equation, the temperature of the fluorescent species can be determined based on the decay time of its fluorescent emission.
The prior art discloses instruments which have attempted to utilize this relationship in order to measure temperature. For example, U.S. Pat. No. 4,542,987 (Hirschfeld) discloses a temperature-sensitive optrode wherein a fluorescent solid composed of solid state laser materials or glass, doped with rare earth metals, is attached to one end of a fiber optic with either a connector or cement. A light source is attached to the other end of the fiber optic. The fluorescent solid is then exposed to heat. Light from the light source propagates through the fiber optic to the fluorescent solid. When the light source is shut off, a fluorescent emission is given off which propagates through the fiber optic to a detector. An electronic processor then converts the decay time of the emission into a temperature.
This apparatus however has inherent limitations in its ability to measure high temperatures, i.e. greater than 500.degree. C. In the preferred embodiment of the '987 patent where the patentee is explaining the use of ruby as the fluorescent source, he specifically states:
Although the response of ruby extends out to about 1000.degree. C., the current practical limitation for operability is about 500.degree. C. The limitation arises primarily from the lack of available high-temperature cements for the interface between the ruby optrode and the long-distance fiber. (Column 8, lines 50-55.)
U.S. Pat. No. 4,673,299 (Dakin) also discloses a fluorescence based temperature sensor using a fluorescent source coupled to a fiber optic, which in turn, is coupled to a light source and detector. However, like the '987 patent, a connector is used to attach the fluorescent source to the fiber optic. Hence, applicant believes the utility of the invention disclosed in this patent also suffers from the same limitation that afflicts the '987 patent, i.e., it cannot measure temperatures greater than the decomposition temperature of the coupling means (either cement or a mechanical connector) connecting the fluorescent source and the fiber optic.
In U.S. Pat. No. 4,997,286 (Fehrenbach, et al.), glass solder is used to connect a glassy optical waveguide to the fluorescent source. As a result, the glass solder is also subjected to very high temperatures. Applicant believes that high temperatures could degrade the glass solder as well as the glassy optical waveguide to which it is attached, and hence destroy the integrity of the device.